It is known that the monomers can be polymerized by using a catalyst system comprising: a compound of a transition metal such as titanium in the trivalent or tetravalent state, magnesium ethoxide as a support material and a co-catalyst of the organo-metallic type, most frequently an organo-aluminum compound.
Although these catalytic systems are active they sometimes result in the formation of polymers containing transition metal more than 100 parts per million by weight. For most of the applications of such polymers, it is essential to remove such catalytic residues by a special treatment.
It is also known that it is possible to increase the catalytic activity of a catalyst by pre-activation. This pre-activation treatment involves contacting the transition metal compound with magnesium and one or more alkyl halides. The pre-activation step results in a catalyst which produces polymers having acceptable physical characteristics. Further, the polymers obtained by using a pre-activated catalyst are capable of being processed by injection molding or extrusion. However, polymers obtained by using pre-activated catalysts have unacceptable residues, which need to be removed.
Conventional magnesium-titanium type Ziegler-Natta catalysts use non-morphological magnesium ethoxide precursors of 700-800 micron size. The catalysts produced using these precursors result in irregular shaped particles. Also, the polymers produced using these catalysts possess particles of irregular shape, low bulk density and a broad particle size distribution.
Shape regularity and size distribution of polymers are dependent on shape and size of the catalyst particles as well as on the components from which the catalyst particles are synthesized. Polymers having regular shape and narrow particle size distribution are desirable for good flowability during extrusion. Several attempts have been made to prepare a catalyst system which is capable of producing polymers having regular shape, narrow particle size distribution and high molecular weight.
WO2005/044873 suggests a method for synthesizing spherical magnesium alkoxide particles by reacting magnesium with alcohol mixture in the presence of iodine at a temperature below the boiling point of the mixture.
US2011/0054129A1 suggests a process for the synthesis of spheroidal magnesium alkoxide particles by reacting magnesium metal, in the presence of iodine, with a mixture of alcohols.
US20040266609 suggests a process for the preparation of a pro-catalyst. In the process, magnesium metal and ethanol in the presence of iodine are heated to obtain magnesium ethoxide. The magnesium ethoxide is then treated with silicon tetrachloride, di-n-butyl phthalate and titanium tetrachloride, and stirred at 125° C. to obtain the pro-catalyst.
US20090203857, US20110054129, U.S. Pat. No. 5,556,820, WO2012007963, US20080281059 and U.S. Pat. No. 5,498,770 suggest a process for the preparation of magnesium alkoxide using magnesium metal and at least one alkanol in the presence of iodine. The magnesium alkoxide obtained is then used for the preparation of a pro-catalyst for Ziegler-Natta catalyst.
However, the spherical magnesium alkoxide particles synthesized by the method of the above processes are frangible and do not retain their morphology or particle size during the synthesis of the pro-catalyst, especially when the pro-catalyst synthesis is carried out on a large scale. Further, the particle size distribution of the magnesium alkoxide particles synthesized by the above mentioned processes need improvement.
The only process that prepares magnesium alkoxide from magnesium metal and alkanol in the absence of iodine is disclosed in U.S. Pat. No. 6,297,188. However, this process suffers from a drawback that only 40 w % of the coarse grains have a screening fraction of ≤500 μm.
Accordingly, there is a need for a process for preparing a shape controlled magnesium ethoxide that retains its shape not only during the synthesis of the pro-catalyst but also during the preparation of the Ziegler-Natta catalyst and the polymerization of monomers.